I don't think there are any particular weapon types for which there is no qualified European alternative. Very many systems are however designed around some amount of American components. Even if there are locally produced, reasonably equivalent versions of those components, you can't just swap them out without major redesign work.
For example the license-manifactured jet engines used in the Saab 39 Gripen. If Trump/Musk pulls the plug on support for those, it will be an epic headache to rebuild around some other engine. Not quite designing a new plane from scratch, but very major rework.